1. Field of the Invention
The present invention relates to a transfer apparatus, and more specifically, to a transfer apparatus for use in an image forming apparatus such as an electrographic copying machine, a facsimile apparatus and a printer apparatus.
2. Description of the Prior Art
FIG. 1 is a front view schematically showing the arrangement of a relevant portion of an image forming apparatus employing a non-contact type transfer roller system. This image forming apparatus is of a clamshell structure constituted by an upper part U and a lower part D. In the upper part U, a photoreceptor drum 1 is provided. In the lower part D, a roller 2, a transfer roller 3 and a pre-transfer guide 4 constituting a transfer apparatus TP, and a conveying guide 15 are provided. In the clamshell structure, since the upper part U is separated from the lower part D to open the conveying path of a recording sheet P, the removal of a jammed sheet and the repair are easy.
On the photoreceptor drum 1, a toner image has been formed by attaching charged toner thereto in advance. The transfer of the toner image to the recording sheet P is performed by the transfer apparatus TP. This is done by applying a voltage of a polarity opposite to that of the toner to the transfer roller 3 when the recording sheet P is fed in the gap between the photoreceptor drum 1 and the transfer roller 3. In the non-contact type transfer roller system, the transfer roller 3 is arranged to face the photoreceptor drum 1 with a predetermined gap between. This gap is formed by arranging at each end of the transfer roller 3 the roller 2 of a diameter slightly larger than that of the transfer roller 3. The roller 2 is in contact with the photoreceptor drum 1 outside of an image formed area on the surface of the photoreceptor drum 1.
The pre-transfer guide 4 guides the recording sheet P to the gap between the photoreceptor drum 1 and the transfer roller 3. The roller 2 and the transfer roller 3 are attached to a bearing (not shown) of a case (not shown) to which the pre-transfer guide 4 is attached, so that the roller 2, the transfer roller 3 and the pre-transfer guide 4 are in positioned condition.
The roller 2, the transfer roller 3 and the pre-transfer guide 4 are pushed toward the photoreceptor drum 1 by a spring 11 provided therebelow (provided on a base 7). For this reason, the roller 2 is pressed against the surface (portions other than the image formed area) of the photoreceptor drum 1, so that a predetermined gap is maintained between the photoreceptor drum 1 and the transfer roller 3. As a result, the transfer apparatus TP is displaceable so that the positions of the transfer roller 3 and the pre-transfer guide 4 depend on the position of the photoreceptor drum 1.
The reason why the transfer roller 3 and the pre-transfer guide 4 are pushed to be displaceable so that a predetermined gap is maintained between the photoreceptor drum 1 and the transfer roller 3 and that the positions of the transfer roller 3 and the pre-transfer guide 4 depend on the position of the photoreceptor drum 1 is that the Upward and downward variation of the position of the photoreceptor drum 1 readily occur in the clamshell structure in which the upper part U and the lower part D are separable from each other.
The recording sheet P on which the image has been transferred by the transfer roller 3 is conveyed from the transfer apparatus TP to the conveying guide 15. The conveying guide 15, which is fixed to the base 7 at its lower part, receives at its end portion 15a the abutment of an end of the image transferred recording sheet P, and guides the recording sheet P to a fixing apparatus (not shown) arranged on the downstream side thereof.
Referring now to FIGS. 2A and 2B, an operation of the transfer apparatus TP performed when the photoreceptor drum 1 moves upward and downward will be described. FIG. 2A shows a positional relationship among the recording sheet P, the transfer apparatus TP and the conveying guide 15 when the photoreceptor drum 1 moves upward (in the direction of arrow mU). FIG. 2B shows a positional relationship among the recording sheet P, the transfer apparatus TP and the conveying guide 15 when the photoreceptor drum 1 moves downward (in the direction of arrow mD).
As shown in these figures, even if the photoreceptor drum 1 moves upward and downward, since the roller 2, the transfer roller 3 and the pre-transfer guide 4 are pushed by the spring 11 (FIG. 1), the positions of the transfer roller 3 and the pre-transfer guide 4 relative to the photoreceptor drum 1 do not vary.
However, since the conveying guide 15 is fixed to the base 7, the position of the conveying guide 15 relative to the photoreceptor drum 1, the transfer roller 3 and the pre-transfer guide 4 varies. Specifically, when the photoreceptor drum 1 moves upward, the distance between the photoreceptor drum 1 and the end portion 15a of the conveying guide 15 increases (FIG. 2A), and when the photoreceptor drum 1 moves downward, the distance between the photoreceptor drum 1 and the end portion 15a of the conveying guide 15 decreases (FIG. 2B).
When the distance between the photoreceptor drum 1 and the end portion 15a is thus varied by the upward and downward movement of the photoreceptor drum 1, the position at which the end of the recording sheet P abuts the end portion 15a is varied. Thereby, the position of the recording sheet P relative to the photoreceptor drum 1 and the transfer roller 3 is varied, so that the transfer to the recording sheet P is unstable to cause non-uniformity in density. Further, since the abutment position of the end of the recording sheet P on the end portion 15a varies, if the end of the recording sheet P is not stably guided, the recording sheet P runs against the end portion 15a to cause paper jam in the worst case.
The following three conditions must be satisfied in the process of conveying the image transferred recording sheet P from the transfer apparatus TP to the fixing apparatus. First, the recording sheet P should be separated from the surface of the photoreceptor drum 1 without being wound up around the photoreceptor drum 1 (i.e. an excellent separating capability). Second, the recording sheet P separated from the surface of the photoreceptor drum 1 should be conveyed without any gap formed between the sheet P and the upper surface of the conveying guide 15 (i.e. an excellent conveying capability). Third, a clear image should be secured after fixing without the transferred toner being scattered on the recording sheet P (i.e. no image scatter).
However, since the conventional conveying guide 15 is made of a material (e.g. acrylonitrile-butadiene-styrene copolymer (ABS) resin material having a comparatively high electric resistance) with which the excess charge does not readily flow from the charged recording sheet P, the separating capability is inferior.
Further, if the excess charge does not readily flow from the charged recording sheet P, a gap is formed between the upper surface of the conveying guide 15 and the recording sheet P separated from the surface of the photoreceptor drum 1, so that the conveying capability deteriorates. If a gap is formed between the recording sheet P and the upper surface of the conveying guide 15, the recording sheet P comes in contact with the upper part U located above the conveying path so that the toner image on the recording sheet P is rubbed. As a result, non-uniformity is caused in the image.
Conversely, when the conveying guide 15 is made of a material (e.g. an ABS resin material having a comparative low resistance and a metal) with which the excess charge readily flows from the charged recording sheet P after the transfer, since the charge rapidly flows from the recording sheet P, the toner transferred to the recording sheet P scatters.
Thus, since the separating and conveying capabilities of the image transferred recording sheet P and the image scatter on the recording sheet P depend on the electric resistance of the conveying guide 15, it is difficult to satisfy all of the above-mentioned three conditions.